INDUSTRIAL AND ENGINEERING CHEMISTRY
748
pears to substantiate this conclusion. Some glasses have been found which give indications of the presence of lines in their x-ray spectrum which might be ascribed to cristobalite, but such lines are weak and not highly diagnostic, and give no suggestion that the other constituents of the glass could be present in groupings even suggestive of crystalline compounds. Other glasses are to be found which give rise to broad and hazy lines suggestive of crystals of colloidal dimensions, but the lines so observed do not correspond to the compounds which would separate if devitrification had taken place. For example, a glass of the composition of the oompound Naz0,2Ca0,3SiOz, the x-ray spectrum of which has been described, and which melts congruently a t 1294" C., gives rise to an x-ray pattern of the type described. Some of this glass was slowly devitrified by long-time heating a t 500" C., and the change in pattern was observed. The new pattern had no relation to the previous hazy lines, and the beginning of crystallization could be observed with the microscope long before a change in the x-ray pattern could be observed. This glass has been crystallized as low as 400' C., and the same crystals separate a t this temperature as a t the liquidus. Most glasses, however, do not give rise even to hazy lines, but give the broad, diffuse band characteristic of the liquid pattern, which has not yet been explained satisfactorily. All lines of evidence point to ,glasses being materials which satisfy the phase equilibrium definition of liquids, which have cooled with continuous change in properties until they have attained the rigidity characteristic of ordinary glass.
Vol. 2.5, No. 7
With respect to the constitution of glass we have little definite knowledge. That such should be the case is not surprising. Glass is a complex solution containing many components, the compounds between which are characterized by a large degree of dissociation in the liquid phase, and as such the problem of its constitution is a more complicated one than that of the constitution of a solution of a salt in water. That is a problem which has attracted the attention of chemists for the past fifty years, and the net result of all their experimental work may be summed up in the statement that we know of no method of establishing beyond a reasonable doubt the constitution of the simplest aqueous solutions. When an adequate theory of the constitution of liquids is formulated, it may be expected to explain the properties of all solutions, not only the solutions of salts in water but also the more complex undercooled solution which is glass. LITERATURE CITED (1) Morey, G. W.,and Bowen, E. L., J. Phys. Chem., 28, 1167 (1924); Kracek, F. C.,Zbid., 34, 1583 (1930). (2) Morey and Bowen, J . SOC.Glass Tech., 9,226 (1925); Morey, J. A n . Ceram. SOC.,13, 683 (1930). (3) Morey, Kracek, and Bowen, J . SOC.Glass Tech., 14, 149 (1930); 15, 57 (1931). (4) Keumann, 2. angew. Chem., 38, 766 (1925); 40,963 (1927). RECEIVED April 11, 1933.
Critical Temperatures in Silicate Glasses J. T. LITTLETON, Corning Glass Works, Corning, X. Y.
S
ILICATE glasses only are to be treated here, not because the organic plastics, which are often included with the silicates, are of insufficient interest, but because it has not as yet been shown that there is any similarity in these two families of materials other than that of appearance. Probably their molecular structures are widely different The question of the existence of any abrupt or abnormally rapid changes in the physical properties of materials as the temperature is changed is often of considerable practical importance. Extrapolation into unexplored temperature regions is never a safe procedure, even with materials of simple composition, but becomes particularly hazardous with materials of several components and of a complex constitution such as the common glasses. Critical temperature points, however, are perhaps of even more interest from the theoretical point of view than from the practical. Any discontinuity or abrupt change in the properties of a material is usually assumed to be associated with, or caused by, a change in the constitution of the material and often a study of such effects will lead to a fairly clear picture of its molecular constitution. The early work on glasses of the Bureau of Standards, particularly the work of Peters and Cragoe and of Tool and his coworkers first pointed out the fact that a change in the con.stitution of glass occurs in the neighborhood of the annealing zone. These observations have been frequently interpreted by others as proving the existence of critical temperatures associated with a discontinuous change in the physical properties of glasses. Accordingly recent glass literature will be Teviewed with the thought of identifying and establishing any real critical temperature points, or to indicate the insufficiency of the data to lead to positive conclusions.
.
The search for critical temperatures in glasses is, however, not so simple as it might appear to be. Berger in a series of recent papers has massed a large amount of information proving almost assuredly that the physical constants of glasses depend to a certain extent upon their past thermal histories. We can suppose that a t any temperature there is a certain equilibrium configuration of a minimum potential energy, but, owing possibly to the highly viscous nature of glass a t low temperatures, these configurations do not follow temperature changes rapidly, but seriously lag behind. This is often spoken of as a freezing-in of the properties of a previous state. At low temperatures the glass may be said almost never to have the properties corresponding to its final state of equilibrium a t this temperature. Yet the changes are so extremely slow, because of the practically solid condition of the material, that these properties may be called unchanging, even though the evidence is that there exists a tendency to change. At intermediate temperatures-that is, in the neighborhood of the annealing temperature-the glass is sufficiently fluid so that the changes of properties with time are rapid enough to be observable. These changes depend upon the past thermal history of the glass. A glass suddenly cooled from a higher temperature level tends to retain the properties peculiar to that temperature state, and considerable time may elapse before the new equilibrium configuration is attained. This change of state or of constitution, taking place during the attainment of equilibrium, is not associated with a critical temperature. The physical properties will change when the glass is brought to a temperature either higher or lower than its previous effective equilibrium level, and hence both the magnitude and rate of change depend upon the magnitude of the change of temperature level and the yiscosity of the glass a t the particular instant of
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1933
observation. If the change in temperature level is large and the rate rapid, these c h a n g e s in properties may be observed with sufficient s u d d e n n e s s a t some temperature so as to make it appear as a critical temperature. Yet if the effect is merely one of retarded change of level, this should not be c o n s i d e r e d as a critical temperature. The distinction b e t w e e n time lag effect and true t e m p e r a t u r e effect is f r e q u e n t l y not made a t all and results in much confusion. THERMAL EXPANSIOK The expansion of metals and many m a t e r i a l s can be represented by a simple cubic equation of the form: dl = cut
where dl t
+ Pt2 +
7t3
change in length per unit of length = temperature increase =
There are no critical temperatures in the properties of glasses. There is a change of some sort, possibly in degree of dissociation or a molecular uggregation which occurs at all temperatures until a stabilization has taken place. This change is resisted by the viscosity of the glass. At low temperatures the retardation is so great thut the change is never measurable in finite times; at intermediate temperatures it is observable, and at high temperatures it is SUBciently rapid so as to completely follow the temperature. The changes in expansion coefic ien t, spec i$c grac ity , electrical conductivity , and heat absorption or spec$c heat are changes in the properties of the material attending these changes in state. At or around the annealing temperature the glass is suficiently jluid for these changes to be obseraable during the course of the measurements i f the rates of temperature change are rapid. I f , however, the rate of temperature change had been slow enough so that at all times the glass was in a state of equilibrium, there would haae been no apparent discontinuities in the data but only a gradual change as the equilibrium condition of the glass changed. The question as to what this change in condition, dissociation, or molecular aggregation m a y be is still unanswered.
The expansion change follows a smooth c u r v e w i t h o u t any discontinuities. G l a s s expansions apparently follow some such law up to a certain temperature. Peters and Cragoe (12) have s h o w n that at a certain temperature, roughly corresponding to the annealing temperature of the glass, there is a sudden rapid increase in the expansion of a glass. Klemm and Berger ( 6 ) and English, Howes, Turner, and Winks (3) state that the expansion consists of four, or fewer, definite linear regions, the curve being made up of intersecting straight lines. These authors are inclined to view the points of intersection as critical points. Tool, Lloyd, and Merritt (17) have further studied the effect of strain. I n Figure 1 is shown a typical expansion curve of a glass-Turner and Winks glass 755K (19). The socalled critical points are indicated on the curves. The changes in slope a t a and b are slight, while a t c, corresponding to the Peters and Crrtgoe point, the change is very pronounced. Figure 2 is also from Turner and Winks and is drawn so that the change points are more in evidence. The authors argue that to represent the data by a smooth curve would be a distortion of the facts. The fact that Klemm and Berger observed similar change points using a different type of rneasuring equipment, and that Turner and Winks did not observe such points on all of their glasses would indicate that these change points are of real significance. Of the 134 glasses cited, 15 show no change points, 5 show one point, 32 show two, and 82 show three. There seems to be no relation between these change points and the composition of the glass. However, they orcur roughly a t similar temperatures in glasses of widely varying composition. Littleton (11) has discussed the justification of these lower temperature points and concluded that thermal expansion can be represented by nieans of the cubic equation within the possible limits of error of the measurements. In the case of glass 814H the expansion can be given by the
749
c o n s t a n t s a = 7.66 X /3 = 3.44 X IOp9,and y = 0. h comparison between the computed results aiid the measurement follows: d l X 10s Computed Obsd.
DEVIATION
% 7.91 12.12 16.50 21.06 25.78 30.68 35.74
7.94 12.09 16.56 21.12 25.88 30.66 35.71
-0.40 +0.25 -0.36 -0.28 -0.39 +0.09
+0.08
This would i n d i c a t e that the cubic equation nearly accords with the observations. Figure 1, curve 2, shows the expansion of a strained glass. At about 400' C. a permanent v o l u m e change begins to appear. The Peters and Cragoe point is c o m m o n l y c a l l e d t h e Ct PO i n t , transformation point, or critical temperature. This is often used to define the annealing temperature. Such points are common in organic glasses and h a v e b e e n t h o u g h t to show a change in the state of c o n s t i t u t i o n of the glass, to i n d i c a t e a c h a n g e from the solid or glassy state to n viscous l i q u i d s t a t e . H o w e v e r , as p o i n t e d out by T u r n e r and Rinks, annealing takes place a t a much lower temperature than the C t point as is shown to be the case in Figure 1. Consequently, this point does not correspond to a change point in the sense that glass changes from a solid to a liquid a t this temperature. Furthermore, not all silicate glasses show such points. Accordingly it can be assumed that these changes are associated with the particular glass composition, rather than a change of state of glassy materials from liquid to solid. It has been shown repeatedly that the density of a glass depends upon its thermal history, and that a specific or stabilized density exists for each temperature. Suppose a glass of an annealing point of 500' C. has been held a t this temperature for some time until it has become stabilized. If, then, it is rapidly cooled, it tends to maintain the density corresponding to the 500" C. condition. On reheating, say to 475", the glass expands normally until it reaches a condition where its state begins to change to the one corresponding to 475'. This volume change is then superposed on its thermal dilatation with the result that the observed changes in length show a deviation from the true thermal expansion curve. If this rate of temperature increase had been much slower-that is, days per degree-at temperatures 200' below the annealing point so as to allow stabilization to take place, then no doubt there would have been a gradual change up to the temperature where the glass becomes sufficiently viscous to yieki. This being the case, the effect may be called a delayed viscosity change. I n order to show further that the bend as observed by Peters and Cragoe depends upon the heating rate, the expansion curves of a lime glass are given. An interferometer for measuring elongation was used in making these observations.
0 4
750 60-H
I N DUSTRIAL A GLASSNQ 7 5 S K
r
N D E K G I I\; E E R I iY G C H E M I S T R Y
t GLASS
55- F
1-01. 25, ?io. 7
about the same in the liquid state, t h o u g h g r e a t l y different in the solid. This work opens a field worthy of much more study. Glasses of a wide variety of properties should be m e a s u r e d , and, if possible, the results correlated. There seems to be no positive evidence of any critical t e m p e r a t u r e s existing on the thermal expansion curves of glasses. The Ct point is not a definite point but might better be called a transformation "zone" or "region," where the equilibrium process is retarded by the high viscosity of the material.
Ne8/4H
VISCOSITY zoo 3w 400 FIGURE2. THERMAL EXPANSION I t would be expected t h a t , if any sudden (2) Chiiled glass CURVE(19) c h a n g e of s t a t e in the c o n s t i t u t i o n of glasses o c c u r r e d in some n a r r o w temFigure 3, curve 1, gives the normal expansion of an an- perature region, such a change should be accompanied by a nealed glass heated a t a rate of 2" C. per minute. The trans- corresponding change in viscosity. Lillie and others have formation occurs at about 510". Curve 2 is the expansion furnished accurate information on the viscosity of glasses curve for the same specimen except that a t about 480" C. the extending over a range of 1000" C. or l O I 5 poises. The obtemperature is held constant for 5 hours. Pigure 4 shows the servations have often been analyzed in an attempt to deduce change of length with time. This glass is cooled from the some law or mathematical equation which would generally temperature of 520" C. after the first run sufficiently slowly express the viscosity as a function of temperature. Such an to be well annealed, but still fast enough so that the proper- equation should partially interpret the physical changes taking place. The viscosity-temperature relationship in some glasses has been recently reviewed by Waterton (20) who considers the equation deduced by Andrade ( 1 ) : l (1) Annealed glass
0
100
7 = Aeb/T
where7 T
FIGURE3.
THERMAL E X P A K S I O N CURVESFOR LIMEGLASS
ties of the 520" states are retained. When held a t 480" it tends to become stabilized a t this temperature. This process is nearly completed in 5 hours. It is evident from this that the expansion change observed by Peters and Cragoe will take place a t no fixed temperature, but depends upon the past thermal history of the glass and the rate of heating of the test specimen. The variation in density with temperature is proportional to the variation in cubical expansion; hence, measurements of density changes can be used as measurements of expansion. Hanlein ( 5 ) has determined the density of glasses a t high temperatures by means of weighing the glass in a platinum crucible suspended in molten salt baths, the density of which had been determined. The results are shown in Figures 5 and 6. There is no evidence of any critical temperature on this curve, except near the annealing temperature, called by Hanlein the transformation point. From the preceding discussion it is apparent that the change a t this point is not the abrupt discontinuous change that Hanlein has shown, but that there is a large though gradual change in volume as the glass passes from the solid to the liquid state. The expansion coefficient changes to a much higher value in this transition region. The change in coefficient with temperature apparently is not greatly different in the liquid from that in the solid state. It is especially noticeable that the expansion coefficients of the two glasses given in Figures 5 and 6 are
= =
viscosity
absolute temperature
This equation was deduced on the assumption of simple viscous liquids completely dissociated or else having a constant degree of association. This means that the relationship between log 7 and l / T should be linear. A departure from linearity is taken to denote a change in the degree of association. If such a departure occurs a t a definite temperature, this point is a critical temperature having some physical significance. Waterton uses data by Lillie (8) and some others, and tests the application of v a r i o u s equations to these measurements. Lillie gives viscosity data on some glasses from about 480" w49 to 1400" C. The data a t low m7 temperatures were obtained by measuring the rate of elongamj tion of a fiber of glass approxi0943 m a t e l y 0.6 mm. in diameter CMJ under v a r i o u s known loads. As the temperatures r e r e decreased, the l o a d s were inFIGURE4 C H A N G E O F creased in steps. Finally, the ZEVGTH WITH T ' I m elongation became too slow to measure under the maximum loads the test specimens would stand. The data on three glasses for this portion of the curve are reproduced in Figure 7. The softening point, denoted in Figure 7 by the S. P. IeveI, is 1 Andrade in a later paper [Nature. 126, 580 (1930j1 calls attention to some earlier derivations of this equation, mentioning Guzman [Anale$ A O C espaii fis. quim., 11,353 (1913j1,Drucker, [ Z . phvszk Chem , 9 2 , 287 (1918) and Dunn [Trans Faraday SOC.,22, 401 (1920)l Thm equation and its derivation aud application have been more completely discussed by Bheppard and Hauck [J Rheol , 1, 349 (1930)l.
I,
July, 1933
I N D U S T R I A L A N D E N G I N E E R I N G C H E ?1 I S T R Y
that temperature at which a standardized specimen under certain specified conditions will elongate under its own weight a t the rate of one millimeter per minute. From the softening point upwards a modification of the rotating cylinder method of hfargules was used ('7). The constants of the equipment
Figure 5
Figure 6
FIGURES5 AND 6. VARIATION OF THERMAL EXPANSION COEFFICIENT AND DENSIT~ WITH TEMPERATURE
(5)
75 1
I n the Lillie curves in Figure 7 , the observations vary from the graph in a systematic manner. The author states that, on first heating a specimen, the viscosity is much less than on subsequent heatings. Apparently there is some configuration cariied down from the high temperature which demands a time to become stabilized to a state corresponding to the temperature of testing, This, however, is insufficient t o explain why the rate of flow per unit of load suddenly increased as the load was increased. Lillie states this is a characteristic of plastic materials, and the inference is that this effect is due to a plasticity. However, there is available no other evidence pointing to the existence of plasticity. Long-time annealing experiments carried out a t very low temperatures do not indicate the existence of any plasticity. I n order to investigate further the possibility of the softening point corresponding to an aggregation temperature, the data given by Lillie (9) on four different glasses are redrawn, using log 7 and 1/T as coordinates (Figure 10). The data in his paper are extended to include later measurements a t the annealing point and strain point-that is, for log 11 equal to 13.4 and 14.6, respectively. These curves show that the relationship pointed out by Waterton is not true in the case of the borosilicate glass. Had the observations been extended to lower temperatures, possibly a straight-line curve might have been found for this lower region. The bend in the viscosity curve a t the annealing point might be taken to indicate that in these two glasses a change of state had occurred. The borosilicate glass is known to devitrify a t temperatures below the softening point and hence would be expected to show a change somewhere below this temperature. However, too much stress should not be laid on the form of the curve a t these low temperatures, as sufficient time was not allowed for the viscosity to become stabilized when the observations were being taken. Viscosity measurements do not extend to temperatures sufficiently low to indicate whether or not there is a change in viscosity a t the transformation point. This point is somewhere in the neighborhood of the annealing point and is sometimes stated to be a t l O I 3 poises. Considering the fact that the Lillie data extend to 1014.6 poises, the indication is that there is no sudden change in viscosity in the region of the transformation temperature. However, all measurements were made with falling temperatures, and
are obtained from geometrical measurements of the apparatus and appropriate formulas so that the results were obtained entirely independently of calibration factors, The complete curve is given in Figure 8. This plot is made on the basis of log 7 against T oC., which is desirable for practical purposes. Lillie calls attention to a change in the curve near 700" C. as indicating a more rapid increase in viscosity with decreasing temperature than would be expected from the course of the curve a t higher t e m p e r a t u r e s . 15 From the data on the curve it is not apparent that this conclusion is justified. 0 Ann, W.Pt Run8 Waterton (20) has plotted the Lillie data as 5 O ~ ? Z ~ ~ a ~ e d . log 7 and 1/T where T is the absolute tempera+ f00 *. ture as in the Andrade formula (Figure 9). He 0 1sff '* '* calls attention to the fact that just a t the softenB 094 x OJ4 ing point of the glass there is a change from the linear relation between log 7 and 1/T to one of more complex form. After giving some consideraI' tion to the fact that this might be due to a change in the method of measurement existing a t this point, he concludes that the data probably represent true viscosity values, and th:tt the change occurs a t a real critical temperature or critical viscosity. The data cited by him from other o b s e r v e r s show further that a t the low '&-laL- & r W < - W - - _ _ _ - _ - _ _ _a2- - _ _ 1.0 temperaturesthe glass behaves as a true - -- - - - E- - - - - - - - - - 6 L C I =--=&'E - - ---- - - _ _-_-_ _ wu - _ _ - -5oo- - - - - 6 a - _- -600- - -Z-1-?E____ liquid, but, after passing through some temperature, there is a deviation from this condition FIGURE7. LOW-TEMPERATCRE VISCOSITYCURVES (8) Temperature sca!e a, glass 111: b , glass I; e, glass 11, and d , comparmon curvee at rlght. which is ascribed to an association beginning a t this temperature. However, no viscosity data are available to show that the viscosity in this region is different the time factor for viscosity has not been sufficiently conwith a rising temperature from that with a falling tempera- sidered, ture; therefore, if this change is caused by association, then, The conclusion therefore is that viscosity data indicate that on cooling, dissociation must occur a t an equivalent rate. at some temperature the glass changes from a true liquid of "
"
9
~~~
ICl-
,d,
40"
"
"
752
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 25, No. 7
The equation is identical in form with the Andrade equation of viscosity p r e v i o u s l y d i s c u s s e d , Glasses have been found to obey the Rasch and Hinrichsen law from the annealing zone down to room temperature. I n strongly strained g l a s s the constants -4 and b change as the glass becomes annealed. Schonborn (14) has observed a rapid change in the conductivity of glass as the annealing temperature is a p p r o a c h e d . Fulda (4) gives measurements on one glass of the following composition in per cent, from room temperature to 6 1200" C.: 71, silica; 5.4, lime; 12.5, sodium oxide; 8.7, potassium oxide: 2.4, alumina. The complete curve for this glass is shown in Figure 12. The resistance varies linearly with 1/T up to a temperature of approximately 460" C. where there is a gradual transition to a second I 0 , straight line ending a t about 650" C. From 900" to 1200" C. the resistance changes very slightly. De9-a C. 600 800 io00 I200 I"* Considerable i m p o r t a n c e has been given VISCOSITY CURVE FOR GLASS I OF FIGURE 7 (8) FIGURE8. COMPLETE to this first transition -point,. generally called the " t r a n s f o r m a t i o n point." The electrical fixed dissociation to one having a different and changing de- conductivity method has been tentatively adopted for measurgree of dissociation. This temperature does not correspond ing this temperature. Fulda shows that this change occurs to a fixed viscosity point but depends upon the glass com- a t a point in the neighborhood of the change in the exposition. This conclusion is based on viscosity data which pansion coefficient and other physical properties, but he does have not become stabilized. Some new results are presented not call it a critical temperature. At this temperature he found the electrical conductivity by Lillie (IO) on his L1 glass (Figure 11). It is apparent from these curves that viscosity changes with time. The of strained and annealed glass to become the same. He viscosity of the initial fiber corresponds to some upper tem- states that with unstrained glass there is no well-defined perature state and gradually increases with time to a value break in the curve but only a gradual transition from one to corresponding t o that of the test temperature. The glass the other direction. By increasing the rate of heating during used for .the measurements given in curve 1, however, has an measurement, the apparent change can be made more marked. initial viscosity corresponding to the state attained a t 478" C. Figure 13 shows this effect. It is apparent that this point depends upon the rate of and gradually falls to that value corresponding to a temperaheating as follows: ture of 487". If stabilized viscosity data are used, the viscosity curves Rate of heating C./min. 0.5 1 5 9 468 479 493 499 will be materially different from those previously cited, and Temp. change, C. probably there will be no critical temperatures in evidence. Viscosity changes with time just as do the other physical The difference of 11" C. obtained in going from 0.5" to 1 O C. properties of the glass. Apparent discontinuities are due to per minute indicates that a much slower rate than 0.5" per minute would have smoothed the curve still farther. insufficient time allowed for stabilization. Fulda a d v i s e s a rate of heating of 5" per minute as ELECTRICAL CONDUCTIVITY I3 being satisfactory to identify a measurable change point. /2 That all glasses conduct electricity, even a t room temperaWith this p a r t i c u l a r glass I / tures, has been well known for a number of years. This conthis gives a value of 493" C. ductivity, however, is so small that a t these low temperatures IO Thermal a nalysis gives a 9 most glasses are relatively good insulators. As the temc h a n g e a t 475O, and the perature is increased, the conductivity increases until a t high 8 relaxation temperature detertemperatures the glasses become very good conductors. The 7 mined by optical m e t h o d s conclusions from the investigations of glass conductivity may 6 is a t 477". The upper limit be briefly summarized as follows: of the transformation zone 5 appears a t 508Oso that the 4 (1) The conductivity obeys Faraday's law. 493" temperature represents 3 (2) The current is carried by the alkali ions. about a mean value for the (3) The degree of ionic dissociation does not vary greatly 2 with the temperature. region. (4) Annealing increases the resistance of glass by a factor of T h e conclusion from FIGURE9. VARIATION OF 8s much as three, depending upon tho initial condition of the t h e s e data is that there is LOG OF VISCOSITY WITH REsample. C I P R O C A L OF A B S O L U T E no sudden d i s c o n t i n u i t y TEMPERATURE (20) in the e l e c t r i c a l conducRasch and Hinrichsen ( I S ) have deduced on a questionable tivity of a n n e a l e d glass, basis from the van 't Hoff equation the formula: but a slow change, the rate of which probably depends upon the viscosity of the glass. The change lags behind the temp = A@JT perature for the more rapid temperature changes; and, = sp. resistance of glass where p when it does begin, it operates so as to give the appearance T absolute temperature of a critical temperature point. A , b = constants
i
INDUSTRIAL .4ND ENGINEERING CHEMISTRY
July, 1933
FIGURE10. FURTHER DATAON VARIATIOX OF LOG OF VISCOSITY WITH RECIPROCAL OF ABSOLUTE TEMPERATURE
It was mentioned earlier that the resistance curve ovcr portions of its length obeys the same law of temperature change as does viscosity-that is, being proportional to 1/T. The viscosity-resistivity relationship, however, does not hold for the higher temperatures. The resistance according to Fulda seems to approach a constant value while viscosity decreases. Unfortunately, there :we no other complete measurements of conductivity in the literature to compare with those of Fulda. The author accordingly makes use of some unpublished measurements from the Corning Glass Works LaboratoryS2 The results are given in Figures 14 to 17. The data on viscosity from Figure 10 are repeated so that a comparison between viscosity and conductivity can be made. The electrical resistance is not indicated beyond the lower limit of the measurement of viscosity; hence it possibly does not extend sufficiently into the region of the transformation zone to indicate its existence. However, the change a t 900" C. observed by Fulda does not appear to exist. It is believed that this is not due to a difference in composition, as the glass of Fulda was fairly near in composition to the lime glass in Figure 14.
753
If the dissociation of the alkali ions does not vary greatly with temperature, it is strange that resistance is not proportional to viscosity in some more intelligible fashion. Possibly this means that molecular viscosity and finite volume viscosities are two different quantities, not by any means proportional to each other. There is a possibility also that the viscosity of the glass has been given a directional effect by the electric field in such a manner as t o aid the transfer of the current carriers. M This relationship I3 s h o u 1d be investigated 12 farther with more simple 11 glasses, preferably those 10 9 who se phase relations 8 are well k n o w n . The 7 time factor, as affecting 6 the r e s u l t s , should be 5 4 more completely deter3 mined. Viscosity data 2 a t lower temperatures I are needed so as to ex6 8 IO 12 14 16 10 20 22 24 Zb 26 24 32 W tend the range into the FIGURE12. VARIATIOV OF LOG field below the so-called OF SPECIFIC RESISTANCE WITH transformation p o i n t . RECIPROCAL OF ABSOLUTE TEMPERLTURE ( 4 ) Such a study should add much to our knowledge of t h e constitution of glasses. The conclusions from the information on electrical conductivity are as follows: (1) The measurements a t 1o w temperatures do not indicate the existence of t h e c h a n g e p o i n t s shown b the Turner and Winks dermal expansion data. FIGURE 13. EFFECT OF R.4TE OF (2) C o n d u c t i v i t y TEMPERATUREINCREASE ON measurements in d i c a t e TRANSFORMATION POINT TEMthe gradual change in the PERATURE ( 4 ) constitution of glasses in the transformation region, thus substantiating the conclusions from the thermal expansion measurements. (3) Conductivity data indicate a continually increasing viscosity of glass down to room temperature and hence no fixed solidification state. (4) There are apparently no critical temperature zones indicated beyond the transformation point.
HEAT ABSORPTION
FIGURE 11.
VISCOSITY AS FUXCTION OF' TIME .4T
487"
c. (10)
In the temperature region covered, neither the laws of Rasch and Hinrichsen nor those of Andrade are obeyed. The viscosity changes much more rapidly than does the resistivity. In Figure 14 the viscosity changer as the fourth power of the resistance and in Figure 16 it varies as the cube. In the other two glasses the relationship is somewhat more complicated, being given by an equation of the form: log 8
= a(1og p
+ b)
or
p
=
b'vl/a
These electrical conducbivity measurements were made through a fellowship a t Purdue University under the direction of K. Lark Horovitz, and the viscosity data on the same glasses are from the paper by Lillie (91. 2
As a crystal melts it absorbs heat without change of temperature, and this heat is released as the liquid crystallizes. When the melting point is attained, the temperature remains constant until the change of state has been completed. It is possible to delay such changes beyond the critical temperature by supercooling, in which case there is a rise in temperature as energy is given off in the crystallization process until the freezing temperature has been reattained, Such thermal studies when applied to glasses can be expected to 3how whether or not there is any change in the molecular constitution of the material and a t what temperature such a change takes place. A change in the rate of heating of an amorphous substance, denoting an increased ,absorption of heat was first abserved by Day and Allen ( 2 ) . Borax glass both in the solid and in the powdered forms when rapidly heated evidenced an ab-
754
I N D U S T R I A L A N D E N G I N E E R I N G C H E 31 I S T R Y
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Vol. 25, No. 7
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/4
J5
F IGURE 14. RELATIONSHIP BETWEEN VISCOSITYAND ELECTRICAL CONDUCTIVITY OF A LIMEGLASS
FIGURE15. RELATIONSHIP BETWEEN VISCOSITYAND ELECTRICAL CONDUCTIVITY OF A LEAD GLASS
sorption of heat in the temperature zone where a sintering of particles of the glass first occurred. This heat absorption lasted over about a 20" C. temperature range, after which the original rate of heating reappeared. A corresponding evolution of heat on cooling was not observed. The authors were able to explain this absorption of heat by assuming a change of state to exist between the glass and its melt. Inasmuch as they did not find any irregularity to exist in the electrical conductivity of the glass in this same temperature zone, they concluded t,hat possibly they were mistaken in their first observations. However, the work of the last ten years in this field has shown that their first observations and conclusions were correct. Tool and Valasek (18) reported that a t a certain stage in the heating cycle of a glass a definitely measurable heat absorption occurs. Later papers by Tool and co-workers have considerably extended this work to include a wider range of glass compositions with a variety of thermal treatments. It was found that heat absorption occurs a t a temperature slightly above the usual annealing temperature, and above this temperature the glass shows an accelerated heating rate. The phenomena are more pronounced in a mellannealed specimen of glass than in a chilled sample. Often in the heating of a chilled glass there is a slight heat evolution, perhaps extending into the region of absorption.
Figure 18, from Tool and Eichlin ( l h ) , indicates that there is a definite change in the specific heat taking place in the glass. This change starts rather slowly, owing no doubt to the high viscosity of the glass, but proceeds a t an increasing rate as the temperature is increased. I n the case of the chilled glass some of the change has been carried over from the original heating, and consequently the effect is not quite so marked. Considerable modification in the heating curve can be obtained by changes in the previous thermal treatment, depending upon the degree of completeness of the change in the sample preceding the heating measurement. This heat absorption or specific heat change takes place in the same temperature zone as do the changes in expansivity and electrical conductivity. The specific heat changes are so small that, unless the rate of temperature increase is large, they m i l l not be observed. In the regions of high viscosity this change is too slow to be measurable in the time intervals of the test. No doubt, if the sample had been stabilized a t such a temperature and then the specific heat measured, a measurable change would have been observed.
DIELECTRIC COXSTAKT Dielectric constant data on silicate glasses are available only for relatively low temperatures, since the ionic conductivity a t elevated temperatures becomes so great that 0
14 I3 I2
/I 10
9 8
7
6 5 4
3
2 I 6 7 8 9 IO lI I2 I3 I4 I5 FIGURE 16. RELATIONSHIP BETWEEN VISCOSITY AND ELECTRICAL CONDUCTIVITY OF A LEAD-BOROSILICATE
GLASS
FIGURE 17. RELATIONSHIP BETWEEN VISCOSITY AND ELECTRICAL CONDUCTIVITY OF A BOROSILICATE GLASS
ISDUSTRIAL AND ETGINEERISG CHEMISTRY
J u l y , 1933
755
glasses in the temperature zones corresponding to the change points in the thermal expansion curve. LITERATURE CITED (1) Andrade, E. N. da C., Nature, 125, 308 (1930). (2) Day, A. L., and Allen, E. T., Am. J . Sci., 19, 93 (1905). (3) English, S., Howes, H. W., Turner, W. E. S., and Winks, F., J . SOC.Glass Tech., 12, 31 (1928). (4) Fulda, M., SprechsaaZ, 60, 810 (1927). (5) Hanlein, W., Glastech. Ber., 10, 126 (1932). (6) Klemm, A., and Berger, E., Ibid., 5, 405 (1927). (7) Lillie, H. R., J. Am. Ceram. SOC.,12, 505, 516 (1929). (6) Ibid., 14, 502 (1931). (9) Lillie, H. R., J. Rheol., 3, 121 (1932).
, Jw’
T€M< “C. ~
5w’
4W‘
FIGURE 18. SORPTION
I # 630“
HEAT ABCURVES(16)
40
80
/zo
(10) Lillie, H. R , paper presented before the meeting of the Am. Ceram. SOC.,Feb. 14, 1933. (11) Littleton, J. T., J . SOC.Glass Tech., 15, 262 (1931). RELATIONSHIP (12) Peters, C. G., and Cragoe, C. H., J . Optical SOC.Am., 4, 105 *mperaturp,T ,o zm 2.0
0
FIGURE 19. BETWEEY DIELECTRIC CONSTANT
AND
TEhfPER.4TURE
(1920).
(13) Rasoh, E., and Hinrichsen, L. W., 2. Elektrochem., 14, 41
(l5)
1. Chilled lass
2.
a
A n n e a l e i glass
any dielectric properties are completely masked. Such observations should serve as a test for the existence of the low-temperature change points indicated by the thermal expansion data of Turner and Winks (19). Observations by Strutt (15) on three different glasses are shown in Figure 19. The different frequencies a t which the measurements are made are indicated on the curves, and it is apparent that there are no indications of any constitutional changes in the
(1906).
Schonborn, H., 2. Physik, 22, 305 (1924). Strutt, M. J. O., Arch. Elektrotech., 25, 715 (1931). Tool, A. Q., and Eichlin, C. G., J . Optical SOC.Am., 8,419 (1924). Tool, A. Q., Lloyd, D. B., and Merritt, G. E., J . Am. Ccram. SOC.,13, 632 (1930). (16) Tool, A. Q., and Valasek, J., Bur. Standards, Sci. Paper 358
(14) (15) (16) (17)
(1920). (19) Turner, W. E. S., and Kinks, F., J . SOC.Glass Tech., 14, 84 (1930). (20) Waterton, S. C., Ibid., 16, 244 (1932).
RECEIVED March
30, 1933
Chemical Composition of Commercial Glasses DONALD E. SHARP,Bailey & Sharp Company, Hamburg, N. Y.
T
The ramges of composition of a wide cariety glass w i t h t h a t of a n c i e n t . of modern c.ommerciai glasses are giren and Analyses of museum specimens of tion of glass acceptable e q u a l l y to t’he physical ancient glass, a few of which are of the reasons lchy the compositions have been chemist, the organic chemist, the given in Table I,have shown the g r a d u a b dtered from those in use glass technologist, and the engiyears c o m p o s i t i o n to be much the ago are pointed out. Analyses of m a n y of the neer. Manysubstancesdiffering same as that of modern glass. glasses are included, and colored glass, technical Such differences as e x i s t a r e w i d e l y in composition are freglass, and optical glass compositionsare treated principally in the alkali concluently designated as glass because they bear a resemblance in tent, t h e a n c i e n t glasses, as briejly. some r e s p e c t to the material a rule, b e i n g m u c h higher in c o m m o n l y known a s glass. s o d a a n d potash t h a n t h e Synthetic resins, for example, are frequently considered to be modern glass. Other differences can be accounted for by glasses because they are transparent and glossy. However, be- impurities that must have been present in the materials cause of the absence of silica in such materials the glass tech- available to the early glassmakers. nologist is inclined to consider them more as glass substitutes TABLEI. .4SALYSES O F h C I E N T GLASSES(16, 17) than as glass. A consideration of commercial glass composi( I n per cent) tions will be sufficiently comprehensive if it is confined only to 10 2 3 4 5 6 those compositions that contain a substantial amount of silica. i:2le: 63.72 63.86 65.95 66.26 68.48 67.44 3.26 0.70 2.98 Glasses containing silica are produced by the fusion, a t $ : ? $ 1.04 0.65 2.49 0.54 0.67 0.28 0.78 0.91 0.51 relatively high temperatures, of three general types of maCaO 9.13 7.86 6.89 7.09 5.71 4.80 5 . 2 0 4 . 1 8 1 . 3 7 1 . 4 8 5 . 2 8 5.64 terials-(1) silica, ( 2 ) alkaline fluxes, and (3) That may be 20.63 22.66 20.30 19.33 14.95 13.94 ?$, 0.41 0.80 0.96 0.37 2.83 1.93 termed “stabilizing ingredients.” Silica is a primary in... ... 0.97 0.61 ... 0.70 gredient in all the glasses; the alkaline flux may contain soda, cuo ... ... ... 0.95 ... ... ... ... 1.08 ... 0.54 0.84 potash, or both; and the stabilizing ingredient, may be an &$: ... ... ... ... 0.95 1.01 oxide Of Calcium, magnesium, barium, zinc, alumirLum,lead, or a Samples 1 a n d 2 , colorless glass from Tel el Amarna, E g y p t , 1400 E . c.; boron, Or a combination of two or more of these. ’$hen silica 3, colorless glass from Elephantine, E g y p t , 200-100 E . c.: 4, d a r k blue glass from Elephantine, E g y p t , 200-100 B. c.; 5, window glass, about 900 A. D . ; and soda only are fused t’ogether, the product is a glass, but a 6, white blown glass about goo A. D. soluble one. The addition of stabilizing ingredients, however, brings about a tremendous reduction in solubility, so that, In general these old glasses are shown to contain roughly to all intents and purposes, t,he resultant glass is completely 65 to 70 per cent silica, 6 t o 10 per cent calcium and magpermanent. nesium oxides, and 16 t o 23 per cent alkali principally as soda. It is interesting to (compare the composition of present day Such glasses are designated by the glass technologist as sodaHERE is no simple defini-
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